Ubuntu 12.04 LTS Precise Pangolin – gnome-shell does not start

After activating the Screen Keyboard in the Typing tab of the Universal Access dialog in System Settings, my Gnome Shell display freezed. After restarting the X server and logging into my account, the Gnome Shell refused to load. I ended up staring at the default background image and after a while, apport proposed to submit a bug report about a crash in gnome-shell (due to XML input).

In .xsession-errors, I found another hint, that gnome-shell crashed (mind the last line!)

Suspecting that the activated screen keyboard is the problem, start a Gnome Classic (No Effects) session. For this, click on the small wheel icon in the top right corner of your login box. In Gnome Classic, revert your screen keyboard settings in the system settings dialog and deactivate the screen keyboard.